Stenlund (1955, p. 44) reported as follows on years of low snowfall, the opposite condition, which demonstrated the same relationship between snow depth and kills on lakes: "The winters of 1951-52 and 1952-53 were abnormally mild with little early snow. As a result, few wolf-killed deer appeared on the lakes and most deer attempted to outrun wolves in the woods."
Thus it appears that extreme snow conditions in our study area increase the vulnerability of deer to wolf predation in three ways: (1) by causing a decline in the health and nutritional state of some members of the deer population; (2) by hindering the escapability of the deer; and (3) by causing deer to congregate on frozen lakes where wolves have the advantage in running.
SUMMARY
During the winters of 1966-67, 1967-68, and 1968-69, the interactions of wolves (Canis lupus) and white-tailed deer (Odocoileus virginianus) were observed in northeastern Minnesota from aircraft. Snow depth and supporting ability were also measured during these winters, and the ability of wolves to capture deer was compared for a period of usual snow conditions versus a period of extreme snow conditions.
It was found that during February and March 1969, when snow remained from 2.5 to 3.9 feet deep and failed to support running deer, wolves were able to capture deer more easily. This was evidenced by kills that were left partly or completely uneaten, and by a higher rate of predation by radiotagged wolves and their associates.
Although both wolves and deer floundered in the extremely deep snow, the relatively lighter weight-load-on-track of wolves evidently gave them a greater advantage than under the usual snow conditions, when wolves were observed floundering more than deer. This factor, plus a decline in the health and vigor of some segments of the deer population and a tendency for deer to congregate on frozen lakes, where wolves have an advantage, help explain the increased vulnerability of deer to wolf predation during the winters of deep snow.
ACKNOWLEDGMENTS
This study was supported by Macalester College, the Minnesota Department of Conservation, the USDA Forest Service, the U.S. Bureau of Sport Fisheries and Wildlife, and the New York Zoological Society. Pilots John Winship, Pat Magie, Jack Burgess, and Don Murray flew the observation planes during radiotracking. Miss Elizabeth Dayton, Mr. Wallace C. Dayton, and the Quetico-Superior Foundation, all of Minneapolis, financed Mech during the writing of this report.
Thanks are also due L. J. Verme, J. P. Kelsall, and J. M. Peek for their helpful reviews.